Background:Knee osteoarthritis (KOA) is the most common cause of a painful joint, and it is associated withsignificant health economic consequences. About literature, KOA is usually diagnosed according to changes seenon conventional radiography, but the radiographic features of OA do not correlate with its symptoms. Nowadays,ultrasonography is becoming a non-invasive imaging tool for OA in the clinical setting. Thus, the purpose of thisstudy was to evaluate the correlation between musculoskeletal ultrasound (MSK US) and the Western Ontario andMcMaster University (WOMAC) osteoarthritis index findings in patients with primary knee osteoarthritis (KOA).Methods:Between August 2015 and October 2017, 50 patients with a mean age of 46.72 ± 9.12 years who fulfilled theAmerican College of Rheumatology (ACR) criteria for KOA were included. All the patients underwent a clinicalassessment with the calculation of the WOMAC index, and they underwent knee US examination and conventionalradiography (CR). Spearman’s rho was used to assess the association between MSK US findings and the WOMAC index.Results:The mean pain score was 10.08 ± 2.89, stiffness was 3.34 ± 1.72, physical function was 26.26 ± 9.6, and the totalWOMAC score was 39.68 ± 12.83. Forty-seven knees showed radiographic femorotibial degenerative signs. The meanthicknesses of the ultrasound-measured articular cartilage of the medial condyle (MC), intercondylar notch area (IA),and lateral condyle (LC) were 0.23 ± 0.60 cm, 0.33 ± 0.69 cm, and 0.30 ± 0.81 cm, respectively. The US findings alsoincluded suprapatellar joint effusion (50%), medial meniscal extrusion (40%), and osteophytes (70%). A positivecorrelation was found between the mean articular cartilage thickness of the IA, mean pain score, and stiffnesssubclasses of the WOMAC score (r=0.342,p=0.015;r=0.414,p=0.003),respectively.Conclusions:The severity of KOA, based on articular cartilage thickness, showed good correlation with the pain andstiffness subclasses of the WOMAC score.